This invention relates to systems and methods for making measurements in three dimensions of complex objects, and more particularly to methods for making a number of critical measurements of metal can tops at various stages in their formation.
The can top is a particularly illustrative example of the problems involved in three-dimensional measurements of formed products, because of the variety of shapes that are used, and the criticality of certain parameters. Tremendous volumes of these can tops are of course manufactured to meet modern demand, and although the equipment is automatic, wear inevitably results in changes in the formed parts. Thus, as the peripheral edge is curled so as to be properly joinable to a can body, and as the central region is shaped to receive a tab for easy opening, readings must be taken periodically to insure that they are not out of specification. The most critical reading is generally regarded as the depth of the score line which is ruptured as the can top is opened with the pull tab. However, this reading is difficult to obtain with a relatively simple but reliable mechanism, inasmuch as the compressive knife edge which forms the score line leaves an irregular pattern, with slanted sides and sometimes with undulations or small protrusions on the bottom surface, and this shape changes as the compressive knife edge wears.
In addition, readings must be taken of a number of salient features during the process of can top formation. These include the outer dimension, the progressively curled peripheral lip, the thickness of the principal panel, the height of an interior ridge at the periphery, the dimensions of a central bubble which is converted into a shaped button, various dimples and depressions, and the thickness and size of the rivet to which the tab is attached. The need for these and other measurements are described in an article entitled "The EndSpector.TM. System for Automated Inspection of Beverage Can Ends" by Robert L. Jackson et al, presented at the Vision 1986 Conference on June 3-5, 1986, and published in the Proceedings. This system may be the first automated system for making dimensional measurements, since prior to that time and still today many organizations engaged in can top manufacture employ manual, contact-type gauges with the attendant difficulties of inaccurate readings, costly and laborious procedures, the need for many technicians, and the difficulty of compiling the statistical information. An automated system, as pointed out in the article, has the further advantage, however fast it is, of generating data in a form which can immediately be processed and analyzed. The article identifies 13 different measurements for critical parameters, and states that the tear-open score and the tab opening rivet are the most critical parameters since they, along with the rivet diameter, determine whether the can end will open easily but that leakage will not occur. The system described in the referenced article uses machine vision equipment modified and extended for the particular purpose. Thus, in order to make the 13 critical measurements, a combination of five cameras is used, comprising four vidicons and one CCD camera, together with five separate circuit boards for digitizing, storage and processing of image data from the different cameras, three frame-buffer units, a high-speed numeric processor for providing low-level image processing operations, and a host computer with high speed, floppy disk and hard disk storage systems.
The assignee of the present invention has for a number of years been manufacturing machine vision systems for imaging analysis and dimensional measurement of a variety of parts, most often integrated circuit elements or units which must be analyzed for critical parameters. In such systems, an X,Y stage on a massive reference table is computer controlled while a Z-axis camera looks at a part positioned on the X,Y stage and generates image signals which are digitized and then analyzed, as by correlator or template machine techniques, to identify a part, determine precise position, make critical measurements and the like. By computer control of the mechanism, and by the use of autofocusing to measure height, relative dimensions in three axes may readily be determined. However, obtaining measurements of the critical parameters in a can top is not readily amenable to handling by this system, because of the problems of certain measurements, and certain practical considerations. For example, thickness dimensions at critical areas, such as the score residual (or web) and the rivet residual are not ascertainable simply from the Z-axis camera, and the small size and variables involved preclude accurate measurement by machine vision techniques. Angles of curvature, complex shapes and other factors also mean that the image detected by a video camera may be of very low contrast in a critical region, so that accurate measurements are not readily feasible. Furthermore, the costs involved in using correlation and template matching techniques should be avoided if possible in obtaining the significant measurements. Some of the practical considerations reside in the fact that delivery and location of partially finished and completely finished can tops should not require much mechanical handling or costly equipment, despite the fact that different manufacturers will desire different measurements, and also despite the fact that some can tops are of substantially different sizes than others. In the United States there are two basic configurations, but in other countries different shapes and therefore sizes and configurations are used. Also, as a practical matter it should not be required to achieve precise, accurate positioning of the can tops during the measurement process, since this would delay operations and impose additional costs. The "EndSpector.TM." system includes, for this purpose, a stacker, loading robot in the form of a 5-axis pick and place device with a special end effector which holds can ends, and a multi-access positioning system for presenting the can end to the cameras and light sources rigidly fixed on a granite base. In order to make the needed measurements, not only are five cameras used, but also special "structured light sources" which are used in various combinations to provide illumination as needed for the particular situation.
With respect to the critical score residual and rivet residual measurements, the "EndSpector.TM. system" utilizes two high-resolution cameras and the set of structured-light sources to obtain differential readings of using small fields of view. Each of the structured light sources projects a line pattern onto the image, and the line is placed across the score and appears to be offset, at the score depth, by a given amount. The camera image is used in measuring the extent of the offset, and because the light source and the camera have fixed positions relative to each other, the position of the surface at the offset region can be determined, and from this the differential measurement of score or residual thickness can be computed. In order to achieve the necessary precision, the cameras must not only be high resolution cameras but the field of view is limited to approximately 0.02 inches, requiring considerable magnification. Interpretation of the image requires exercise of a correlation function, which substantially increases the cost and complexity of the system.
Clearly a system based on the assignees's pre-existing machine vision system would be far less costly and complex than the "EndSpector.TM." system, if the needed measurements can be provided.